The prospect of being denied engine service under warranty because of using a non-API oil or an oil with viscosity other than what is recommended is a constant theme here. I believe that it drives some decisions about oil choice that may not be optimal, based on driver habit, environment, vehicle usage, etc. Let's see how much of a real-world issue it is here, and how much is it mythology. 1. Have you ever been denied warranty service because you used a non-API oil? 2. Have you ever been denied warranty service because you deviated from the viscosity recommended/required in the manual? 3. Has a dealer ever questioned what viscosity or brand of oil you used for the purpose of determining your compliance with the manual?